1. Field of the Invention
The present invention relates to a controller for an electric vehicle including an inverter which supplies electric power to a driving motor (an alternating current induction motor), and more particularly to a controlling system which suppresses vehicle vibration occurring when power supply from the inverter to the driving motor is shut off.
2. Description of Related Art
Generally, an alternating current induction motor for an electric vehicle is driven by polyphase alternating current converted by an inverter from direct current of a battery mounted on the vehicle. The inverter is composed of plural switching elements which is controlled by pulse width modulation signals generated in a microcomputer. An example of an inverter controller of this kind is disclosed in JP-A-62-48201. In the controller disclosed, in order to suppress surge current generated in the system when an acceleration pedal is suddenly released while the driving motor is rotating at a relatively high speed, the inverter is not abruptly shut off but controlled with a predetermined duty ratio and by pulse width modulation signals which are common to each phase. Since the driving motor is mounted on a vehicle with a resilient material such as rubber bushings, the driving motor vibrates as a reaction when a driving torque of the motor is rapidly decreased. This kind of vibrations is generated when the acceleration pedal is released or a shift lever is shifted from a driving position to a neural position and thereby the switching operation of the inverter is shut off. The vibrations are much detrimental to drive feeling. The controller disclosed in the aforementioned publication which aims at suppressing the surge current is not effective or sufficient to solve the problem of vibrations.